Reef Chemistry Question of the Day #68 Water Changes and Sump Size

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #68

There are many decisions that go into designing a reef tank. One is whether to have a sump, and if so, how large.

This question bears on just one aspect of sump size: how it impacts the accumulation of unwanted ions in relation to water changes.

Assume that you have a system with a total water volume of 100 gallons and perform a daily water change of 1 gallon.

Also assume that it has a net accumulation (before the daily water change) of 0.25 ppm of nitrate each day and that this net accumulation does not change over time, except when a water change lowers is.

After 1 year, nitrate has accumulated to a certain level. Which value is closest to that level?

A. 0.4 ppm
B. 2 ppm
C. 24 ppm
D. 180 ppm

Now assume everything else is unchanged, but you increase the sump size so that the total water volume is now 200 gallons (twice as large).

How is the nitrate level after 1 year with the larger sump compared to with the smaller sump? Assume no impacts on nitrate except as directly relate to the increased water volume via the same total amount of nitrate added each day as with the smaller sump, and the same 1 gallon water changes daily.

E. It is higher because the 1 gallon water changes are less efficient with a larger total water volume
F. It is the same because only the total production of nitrate matters
G. It is smaller because the nitrate is diluted more with a larger total water volume

Feel free to only answer the second part if you want, as that is the main point of the question.

Good luck!

























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[treemanone2003]

I'm doing something wrong with my calculations of part one and come up with a value of slightly less than "C" or slightly higher than "D". (22.8125 or 182.5 respectively)

Part two the answer is "E".

 

[beaslbob]

You're doing 1% water change per day in a system that is increasing nitrates at .25ppm/day.
At the end point or steady state the amount of nitrates removed must equal the amount built up between water changes. In this case .25ppm. If you are removing 1% ofthe water then the just before water change value must be 100 times the dailychange.
(before water change conditions)=(change between water changes)/(1/fraction of water change)
In this case:
The “end point” (steady state) is .25ppm/(1/100)=25 ppm. (just before water changes)
(just after water change is goes down to 24.75 then up to 25ppm before the next day’s water change).
Answer C 24 ppm is the closest.

Increasing the size means the water change is 1/200 per day.
So that end point is .25ppm/(1/200)=50ppm just before the water change.
(down to 49.75 after the water change them back up to 50ppm before the next water change.
answer: C and E.
but then I have been wrong before and perhaps tossing a bunch of number around will confuse everyone.

Dr.randy: How’s that for a rocketscientist. :wink::xd:

 

[beaslbob]

just keep in mind if I'm right the entire reef chemistry of the day thing was shot on the third post. I could be just a plant.

 

[beaslbob]

After all you double the total volume and the nitrates double also. How does that make any sense?

 

[Mike J.]

It goes like this. day 1: .25 - .0025 = .2475, day 2: .2475 + .25 = .4975 - .004975 = .492525, day 3: .73509, day 4: .97524, day 5: 1.21299, day 6: 1.44836, day 7: 1.68138, day 8: 1.91206
ON AND ON .25 - 1% 365 times. The closest answer would be C. And E.

 

[Randy Holmes-Farley]

Lots of good discussion so far.

 

[hart24601]

Ok... so I did the math because I kept thinking about it. 24ppm for the 1st part. For part 2 if the larger size is still producing .25ppm it's 42ppm, but if you half the PPM because the tank is twice the size you get 21ppm. Unless I missed something, I guess they are about the same since total nitrate production is the same.

 

[aslmx]

What's a water change? Do I need to remove it from the sump or overflow thingy?

 

[treemanone2003]

In hindsight I never actually answered part one. My first choice was "C" as that appeared to fit best, however, since I didn't physically write that down, do I still get 1/2 credit IF part 2 is correct ?

 

[Randy Holmes-Farley]

In hindsight I never actually answered part one. My first choice was "C" as that appeared to fit best, however, since I didn't physically write that down, do I still get 1/2 credit IF part 2 is correct ?

:lol:

Yes, but only if part 2 is correct.

 

[fishroomlady]

C and G

 

[gtbarsi]

C
G When you double the water volume the nitrate increase is cut in half and starts increasing at a rate of .125 ppm.

What is interesting to note is that 1 gallon / day is not enough of a water change regardless of the system size to reach a stable concentration of nitrate. It stretches out but at the two year mark the values are still increasing for both systems. The 100 gallon system at about 24.73 ppm and the 200 gallon system at 24.23 ppm.

If you increase the water changes to 2 gallons / day on the 200 gallon system the rate of increase is cut in half but still is increasing at the end of two years. The 100 gallon system 2 gallons / day cuts the concentration in half and comes very close to finding an equilibrium between 12.24 at the end of 1 year and 12.25 at the end of two years.

At 3 gallons / day the 200 gallon system seems to have come close to stable at 8.179 at the end of 1 year and 8.212 at the end of 2 years. The 100 gallon system has found equilibrium at 8.0833333

This makes me want to start charting my nitrates over a much longer period of time. From there I can deduce the trend years in advance and see how big I actually need the water change to be to hit a goal concentration, then what it will take to maintain it long term.

Someone let me know if you think my math is completely crazy, I can provide the Excel spreadsheet for anyone that asks.

 

[gtbarsi]

OK, here are some tough questions to answer but I'm going to ask it since I have been thinking about them since last night.

Has anyone mapped their daily Nitrate upswing in PPM?
If so what was yours?

Does anyone have any sense as to the range of daily upswing that is supportable for a Mixed reef or FOWLR tank?

 

[beaslbob]

I still stand by my original answer. Which means i'm stubborn if nothing else. Even with twice the volume nitrates still have to build up to twice what they were before to remove the daily increase. But it does take longer to reach that end point. But 365 days is long enough to be within this engineer's old slide rule accuracy. Or I'm just a crazy plant that doesn't understand anything.

 

[beaslbob]

I guess if the total production of nitrate remains the same then the daily increase is now .125ppm not .25. end point is now .125/(1/200)=25ppm. So no change. So you double your total volume and nitrates stays the same? Trickie trickie trickie.

 

[beaslbob]

dern why do I keep falling for these?

 

[Randy Holmes-Farley]

OK, here are some tough questions to answer but I'm going to ask it since I have been thinking about them since last night.

Has anyone mapped their daily Nitrate upswing in PPM?
If so what was yours?

Does anyone have any sense as to the range of daily upswing that is supportable for a Mixed reef or FOWLR tank?

What do you mean by supportable?

 

[Randy Holmes-Farley]

dern why do I keep falling for these?

:lol:

So many things we do end up being fairly complicated if you really want to understand it fully.

 

[Zero Nitrates]

C and G